Optical transmitter circuit

ABSTRACT

An optical transmitter circuit including a light receiving element, such as a photodiode, which monitors the optical output of a light emitting element such as a semiconductor laser. A current-voltage converting circuit supplies a drive current from a drive circuit to the light emitting element and converts the output voltage of the light receiving element into voltage. An APC amplifier compares the converted output signals and a reference signal, and a hold circuit holds the output signal of the APC amplifier and uses the output signal as a current control signal of the drive circuit. A “1” continuous signal detecting circuit detects the continuation of “1” in a specified number of bits in the input data (DATA) and updates the hold value in the hold circuit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority of Japanese PatentApplication No. 11-75025, filed Mar. 19, 1999, the contents beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical transmitter circuit whichenables reliable monitoring of optical signals generated by a lightemitting element, such as a semiconductor laser, with a comparativelyinexpensive light receiving element, such as a photodiode. Moreparticularly, the present invention relates to an optical transmittercircuit having a circuit to detect continuous “1” bits in the input datain order to perform update timing of a current control signal ordeterioration evaluation of the light emitting element.

2. Description of the Related Art

Optical transmitter circuits are known. FIG. 13 illustrates an exampleof a conventional optical transmitter circuit, including a lightemitting element 101 such as a semiconductor laser, a light receivingelement 102 such as a photodiode to monitor light, a current-voltageconverter (I/V) 103, an auto power control (APC) amplifier 104, a drivecircuit 105, a sample hold circuit 106, and an analog switching circuit107.

In operation of the conventional optical transmitter circuit shown inFIG. 13, the drive circuit 105 supplies drive current to the lightemitting element 101 in accordance with input data (DATA). An opticalsignal is generated by the light emitting element 101 in accordance withthe input data (DATA). The optical signal is detected by the monitoringlight receiving element 102, converted into voltage by thecurrent-voltage converting circuit 103, and input into the APC amplifier104. The APC amplifier 104 compares the output signal of thecurrent-voltage converting circuit 103 and a reference value, and inputsa signal corresponding to the comparative differential thereof into thesample hold circuit 106 via the analog switching circuit 107.

The analog switching circuit 107 receives the output signal of the APCamplifier 104 when the data (DATA) converted to optical signals istransmitted at level “1”, which output signal is input and held in thesample hold circuit 106. The held signal is input into the drive circuit105 as a current control signal, and the drive current of the lightemitting element 101 is controlled so that the optical output remainsconstant.

FIGS. 14A-14D are diagrams explaining the operation of the conventionalexample of the optical transmitter circuit shown in FIG. 13. Morespecifically, FIG. 14A illustrates the data (DATA) input to the drivecircuit 105; FIG. 14B illustrates the optical output signal of the lightemitting element 101; FIG. 14C illustrates the output signal of thecurrent-voltage converting circuit 103; and FIG. 14D illustrates thedrive current supplied to the light emitting element 101.

In the example shown in FIGS. 14A-14D, when the optical output signal ofthe light emitting element 101 corresponding to the data (DATA) that isinput at time t1 decreases, as indicated by the OUTPUT DETERIORATION inFIG. 14B, the output signal of the current-voltage converting circuit103 also decreases, as shown in FIG. 14C. The decreased output signal ofthe current voltage converting circuit 103 is held by the sample holdcircuit 106 via the analog switching circuit 107, and a drive current issupplied to the light emitting element 101 from the drive circuit 105 asa current control signal corresponding to the data (DATA) that is inputat the following time t2. More particularly, as shown in FIG. 14D, sincethe drive current corresponding to the data (DATA) that is input at timet2 is increased more than the drive current corresponding to the data(DATA) input at time t1, the optical output signal is controlled to aspecified level, as indicated by the OUTPUT RECOVERY of FIG. 14B.

Furthermore, a circuit is known in which stabilization of the opticaloutput is achieved by converting the output current of the lightreceiving element, which converts the optical output of the lightemitting element, into voltage with a current-voltage convertingcircuit. When the converted optical output of the light emitting elementreaches a specified level or above, the circuit determines the output tobe a significant detection signal, holds it as a sample, and controlsthe drive current of the light emitting element in accordance with theheld value. An example of this type of circuit is disclosed in JapaneseUnexamined Laid-Open Patent Application Publication JP9-18054, whereinthe input data is delayed and is considered to be a significantdetection signal, and the output signal of the current-voltageconverting circuit at this time is held as a sample.

When data (DATA) that is input is increased in speed, e.g., from a lowspeed of 50 Mbps to 150 Mbps, the light emitting element 101 hassufficient response speed since the light emitting element 101 isgenerally a semiconductor laser. However, a photodiode is generally usedas the light receiving element 102. Because the photodiode has anincreased surface area in order to increase the light receivingsensitivity, the capacitance C_(PD) of the photodiode is generally, forexample, 20 pF or above. Accordingly, when the current-voltageconverting circuit 103 includes a resistance R, the band region f_(O) isdetermined by the equation f_(O)=½πRC_(PD), so it is extremely difficultto broaden the band. In other words, when a comparatively inexpensivephotodiode is used as a light receiving element 102 to monitor the lightemitting element 101, the response characteristics are not sufficient todetect an optical signal having a high speed of 150 Mbps or more.

FIGS. 15A-15D are diagrams illustrating problems occurring with the APCamplifier 104 operation in accordance with the conventional opticaltransmitter circuit. More specifically, FIG. 15A illustrates the data(DATA) input to the drive circuit 105; FIG. 15B illustrates the opticaloutput signal of the light emitting element 101; FIG. 15C illustratesthe output signal of the current-voltage converting circuit 103; andFIG. 15D illustrates the drive current supplied to the light emittingelement 101. When the input shown in FIG. 15A occurs, the sample holdcircuit 106 samples and holds the output signal of the APC amplifier 104when the data (DATA) has been input. At this time, since the responsespeed of the light receiving element 102 is lower than the speed of theinput data (DATA), the output signal of the current-voltage convertingcircuit 103 changes, as shown in FIG. 15C.

Moreover, the current control signal corresponding to the value that hasbeen sampled and held is updated at the times t1, t2, t4, and t6, asindicated by the arrows showing the CURRENT UPDATE VALUE in FIG. 15, andthe drive current supplied to the light emitting element 101 from thedrive circuit 105 is updated. Accordingly, the output signal of thecurrent-voltage converting circuit 103 resulting from the monitoring ofthe optical output produced by the light receiving element 102 inresponse to an initial input value of “1” becomes equal to or less thanthe set value indicated by the broken line at the time t1 in FIG. 15C.Because this output signal of the current-voltage converting circuit 103is sampled and held, it is judged to be a lower optical output than aspecified level, and the drive current supplied to the light emittingelement 101 for the next continued data value of “1” increases, as shownin FIG. 15D. Accordingly, the optical output signal of the lightemitting element 101 at this time becomes greater than the initialoptical output, as shown in FIG. 15B.

In this state, at the following time t3, the drive current correspondingto the input data (DATA) “1” is supplied to the light emitting element101. At this time, even if the optical output of the light emittingelement 101 exceeds the specified level, the output signal of thecurrent-voltage converting circuit 103 is lower than the set value attime t4 because of the response speed of the light receiving element102, as shown in FIG. 15C. By sampling and holding the output signal ofthe current-voltage converting circuit 103, the drive current withrespect to the input data (DATA) “1” at the next time t5 is furtherincreased, as shown in FIG. 15D.

Monitoring the level of the output signals of the current-voltageconverting circuit 103, and determining whether there is deteriorationof the light emitting element 101 when this level falls below aspecified value has been considered. However, since the response speedof the light receiving element 102 is insufficient as described above,there is a problem that a drop in the optical output level of the lightemitting element 101 can be mistakenly assumed.

The above-described problem occurs when a photodiode which does not havea small capacitance is used as the light receiving element 102 inperforming APC control. For this reason, the selection and use of aphotodiode having a capacitance C_(pd) of 2 pF or less as the lightreceiving element 102 has been considered. However, this type ofphotodiode is extremely expensive. Photodiodes having high-speedresponse characteristics are also expensive. Thus, it has been extremelydifficult to achieve cost reduction in the optical transmitter circuitwhich performs stabilization of the optical output and detection ofdeterioration in the optical signals with respect to high-speed data.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticaltransmitter which achieves stabilization of the optical output and thusenables a reliable detection of deterioration of a light emittingelement, even when a photodiode which does not have a small capacitanceis used as a light receiving element.

Objects and advantages of the present invention are achieved inaccordance with embodiments of the present invention with an opticaltransmitter circuit comprising a light emitting element to convert inputdata to an optical output; a light receiving element to receive theoptical output of the light emitting element and to output a currentcorresponding to the optical output; a current-voltage convertingcircuit to convert the output current of the light receiving elementinto a voltage and to output a voltage signal; an APC amplifier tocompare the output voltage signal of the current-voltage convertingcircuit and a reference signal and to output a differential outputsignal; a hold circuit to hold the output signal of the APC amplifierand to form a current control signal; a drive circuit to receive thecurrent control signal from the hold circuit and to supply drive currentto the light emitting element in accordance with the current controlsignal; and a “1” continuous signal detecting circuit to detect aspecified number of continuous of “1” bits in the input data and toperform an updating operation on the output signal of the amplifier heldin the hold circuit.

In accordance with embodiments of the present invention, the holdcircuit may comprise an analog switching circuit to receive the outputsignal of the amplifier and a detection signal of the “1” continuoussignal detecting circuit, and to switch on in response to the “1”continuous signal detecting circuit detecting the specified number ofcontinuous “1” bits; and a peak detecting circuit to detect and hold apeak value of the output signal of the AC amplifier that is input viathe analog switching circuit.

The optical transmitter circuit may further comprise a peak detectingcircuit to detect a peak value of the output signal of thecurrent-voltage converting circuit and to input the peak value to theamplifier.

The optical transmitter circuit may further comprise a first peakdetecting circuit to detect a peak value of the output signal of thecurrent-voltage converting circuit and to output the peak value to theamplifier; and a second peak detecting circuit to detect a peak value ofthe input data and to output the peak value of the input data to theamplifier as a reference value.

In accordance with embodiments of the present invention, the holdcircuit may comprise an up/down counter to count up or downcorresponding to an output signal of the amplifier, in response to the“1” continuous signal detecting circuit detecting the specified numberof continuous “1” bits; and a D/A converter to convert the contents ofthe count of the up/down counter to an analog signal and to input thecontents of the count of the up/down counter to the drive circuit as ananalog current control signal.

Objects and advantages of the present invention are achieved inaccordance with embodiments of the present invention with an opticaltransmitter circuit, comprising a light emitting element to convertinput data into an optical output; a light receiving element to receivethe optical output of the light emitting element and to output a currentcorresponding to the optical output; a current-voltage convertingcircuit to convert the output current of the light receiving elementinto a voltage and to output a voltage signal; a light deteriorationdetecting comparator to detect deterioration in the light emittingelement by comparing the output signal of the current-voltage convertingcircuit with a reference value and to output a deterioration detectionsignal; a “1” continuous signal detecting circuit to detect a specifiednumber of continuous “1” bits in the input data; and a flip-flop to holdthe deterioration detection signal from the light deteriorationdetection comparator in accordance with the detection signal from the“1” continuous signal detecting circuit.

In accordance with embodiments of the present invention, stabilizationof the optical output of the light emitting element and detection ofdeterioration of the light emitting element can be performed.Furthermore, in accordance with embodiments of the present invention,stabilization of the hold value of the hold circuit or evaluation oflight emitting element deterioration is performed by delaying thedetection signal from the “1” continuous signal detecting circuit by adelay circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbecome apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings, of which:

FIG. 1 is a block diagram of an optical transmitter in accordance with afirst embodiment of the present invention.

FIG. 2 is a circuit diagram of the optical transmitter in accordancewith the first embodiment of the present invention.

FIGS. 3A-3D are diagrams explaining the operation of the opticaltransmitter in accordance with the first embodiment of the presentinvention.

FIG. 4 is a block diagram of an optical transmitter in accordance with asecond embodiment of the present invention.

FIGS. 5A-5D are diagrams explaining the operation of the opticaltransmitter in accordance with the second embodiment of the presentinvention.

FIG. 6 is a block diagram of an optical transmitter in accordance with athird embodiment of the present invention.

FIG. 7 is a block diagram of an optical transmitter in accordance with afourth embodiment of the present invention.

FIG. 8 is a diagram of a delay circuit in accordance with embodiments ofthe present invention.

FIG. 9 is a block diagram of an optical transmitter in accordance with afifth embodiment of the present invention.

FIG. 10 is a diagram of a “1” continuous signal detecting circuit inaccordance with embodiments of the present invention.

FIG. 11 is a block diagram of an optical transmitter in accordance witha sixth embodiment of the present invention.

FIG. 12 is a block diagram of an optical transmitter in accordance witha seventh embodiment of the present invention.

FIG. 13 is a block diagram of a conventional optical transmitter.

FIGS. 14A-14D are diagrams explaining operation of the conventionaloptical transmitter.

FIGS. 15A-15D are diagrams explaining the problems of APC operation inthe conventional optical transmitter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the present preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

FIG. 1 is a block diagram of an optical transmitter in accordance with afirst embodiment of the present invention. As shown in FIG. 1, theoptical transmitter includes a light emitting element 1 such as asemiconductor laser, a light receiving element 2 such as a photodiode, acurrent-voltage converting circuit (I/V) 3, an APC amplifier 4, a drivecircuit 5, a sample hold circuit 6, an analog switching circuit 7, and a“1” continuous signal detecting circuit 8.

The light receiving element 2 is a photodiode to monitor the outputcurrent of the light emitting element 1. In accordance with the presentinvention, the light receiving element 2 is preferably a photodiodewhich does not have a small capacitance. For example, the capacitance ofthe light receiving element 2 photodiode may be 20 pF, or approximately20 pF. The monitored output of the light emitting element 1 produced bythe light receiving element 2 is a current which is converted to voltageby the current-voltage converting circuit 3. The voltage output by thecurrent-voltage converting circuit 3 is then compared with a referencevalue by the APC amplifier 4, and the differential output signal of theAPC amplifier 4 is input to the sample hold circuit 6 via the analogswitching circuit 7. The value held by the sample hold circuit 6 is theninput to the drive circuit 5 as a current control signal. Furthermore,the “1” continuous signal detecting circuit 8 detects the continuationof a bit value of “1” in the data (DATA) input to the drive circuit 5.When it is detected that several “1” bits have been generatedcontinuously, the analog switching circuit 7 is switched on, and theoutput signal of the APC amplifier 4 is input and held in the samplehold circuit 6.

Therefore, in accordance with embodiments of the present invention, evenwhen a light receiving element 2 having a slower response speed than thespeed of the input data (DATA) is used, because the light emittingelement 1 converts to continuous optical signals in response to thedetection of continuous “1” bits, the monitored output of the lightreceiving element 2 is also gradually increased to a specified value.The continuous number of “1” bits that is detected to increase theoutput of the light receiving element 2 to the specified value is apredetermined number set in advance, and the output signal of the APCamplifier 4 when “1” has been detected in the predetermined number ofcontinuous bits is held in the sample hold circuit 6. Therefore, if thelight receiving element 2 is a photodiode which does not have a smallcapacitance, the stabilization of the optical output can be achieved inaccordance with the output signal of the APC amplifier 4. Moreover, ifthe hold value of the sample hold circuit 6 falls below a set value, itcan be determined that there has been deterioration in the lightemitting element 1.

In accordance with embodiments of the present invention, a default valueis set as an initial status of the sample hold circuit 6, and a defaultvalue of the current control signal can thereby be input to the drivecircuit 5. The default value can be updated by a continuous “1” bitdetection, as described above.

FIG. 2 is a circuit diagram of the optical transmitter in accordancewith the first embodiment of the present invention. As shown in FIG. 2,Q1 through Q9 are field effect transistors (FETs), and Q7 and Q8represent depression type field effect transistors. Hereinafter, theFETs are referred to simply as transistors. C1 and C2 are capacitors, D1through D7 are diodes, R1 through R6 are resistors, CI1 through CI4 areconstant-current supplies, V_(DD) and V_(SS) are power voltage, V_(REF)is a reference voltage, and G1 a complementary output gate circuit.

As shown in FIG. 2, the current-voltage converting circuit 3 iscomprised of a resistor R6. The output current of the light receivingelement 2 corresponding to the optical output of the light emittingelement 1 is converted to voltage by the resistor R6, input to the APCamplifier 4, and compared with the reference voltage V_(REF). Thedifferential output signal of the APC amplifier 4 is input to the samplehold circuit 6 via the analog switching circuit 7.

The sample hold circuit 6 is preferably a peak detection circuitcomprising a diode D7 and capacitor C2. The sample hold circuit 6 inputsthe terminal voltage of the capacitor C2 to the gate of the transistorQ7 of the drive circuit 5 as a voltage control signal.

As shown in FIG. 2, the “1” continuous signal detecting circuit 8 iscomprised of a constant-current supply CI4, a transistor Q9, and acapacitor C1. The transistor Q9 is switched on in accordance with a “1”bit in the input data (DATA), and the capacitor C1 is charged from theconstant current supply CI4. Transistor Q9 is switched off in accordancewith “0” bit in the input data (DATA). When the transistor Q9 isswitched off, the capacitor C1 discharges via analog switching circuit7, or alternatively via a pathway not shown in the drawing.

In accordance with the first embodiment of the present invention, thedischarge time constant of the capacitor C1 is set corresponding to thetransmission speed of the input data (DATA), and the capacitor C1 isstructured to discharge to under a specified value at least within a1-bit interval of a “0.” When a “1” has continued in the input data fortwo (2) bits or longer, the terminal voltage of the capacitor C1 exceedsthe specified value and is input to the analog switching circuit 7 as adetection signal via a gate circuit or the like (not shown in thefigure), thereby switching on the analog switching circuit 7.

In accordance with embodiments of the present invention, the analogswitching circuit 7 comprises a field effect transistor (FET), and theterminal voltage of the capacitor C1 is applied to a gate thereof sothat the field effect transistor is switched on when the terminalvoltage of the capacitor C1 exceeds a specified level. The “1”continuous signal detecting circuit 8 may also operate such that theterminal voltage of the capacitor C1 is made to exceed the specifiedvalue when “1” has continued for three (3) bits or more, and the analogswitching circuit 7 is switched on only when “1” has continued for three(3) bits or more.

Moreover, the drive circuit 5 includes a gate circuit G1 into which data(DATA) is input, and output signals from a non-inverted output terminalof the gate circuit G1 are input to the gate of the transistor Q1.Output signals from the inverted output terminal of the gate circuit G1are input to the gate of the transistor Q2. The transistors Q1 and Q2make up a differential circuit in conjunction with a constant-currentsupply CI1, which is commonly connected to the source of transistors Q1and Q2. A level conversion circuit, formed by transistors Q3 and Q4 andconstant-current supplies CI2 and CI3, inputs level-converted signals tothe gates of transistors Q5 and Q6, inputs a current control signal tothe transistor Q7 which is commonly connected to transistors Q5 and Q6,and controls the current value supplied to the light emitting element 1via the transistor Q6. Furthermore, the bias current of the lightemitting element 1 is applied via the transistor Q8 in accordance withthe setting of the resistor R5.

FIGS. 3A-3D are diagrams explaining the operation of the opticaltransmitter in accordance with the first embodiment of the presentinvention. More specifically, FIG. 3A illustrates data (DATA) input tothe optical transmitter, FIG. 3B illustrates the optical output signalof the light emitting element 1, FIG. 3C illustrates the output signalof the current-voltage converting circuit 3, and FIG. 3D illustrates thedrive current supply to the light emitting element 1. As shown in FIG.3A, when a “1” bit is input continuously at time t0 and at the followingtime t1, even when the output signal of the current-voltage convertingcircuit 3 does not increase to the set value (indicated by the brokenline shown in FIG. 3C) because of the response delay of the lightreceiving element 2, the output signal of the current-voltage convertingcircuit 3 is able to increase to the set value at the time t2 because ofthe continuation of “1” for two (2) bits.

At this time, the terminal voltage of the capacitor C1 of the “1”continuous signal detecting circuit 8 exceeds the specified valuefollowing time t1. The analog switching circuit 7 is thereby switchedon, and, as shown as the CURRENT VALUE UPDATE at the time t2, the peakvalue of the output signal of the APC amplifier 4 is held by thecapacitor C2 of the sample hold circuit 6. Accordingly, a drive currentroughly equal to the drive current supplied to the data input in theprevious times t0 and t1 is supplied to the data input at the followingtime t3.

When one isolated “1” bit is input at times t3 and t5, respectively, theoutput signal of the current-voltage converting circuit 3 does notincrease to the set level indicated by the broken line at times t4 andt6 because of the slow response characteristics of the light receivingelement 2. Moreover, the terminal voltage of the capacitor C1 of the “1”continuous signal detecting circuit 8 does not increase to the specifiedlevel, and thereby the analog switching circuit 7 remains off.Accordingly, since the output signal of the APC amplifier 4corresponding to the excessive output signal of the current-voltageconverting circuit 3 is not held, the current value is not updated.

When “1” bits are continuously input, the analog switching circuit 7 isswitched on by the “1” continuous signal detecting circuit 8, and theoutput signal of the APC amplifier 4 can be held in the sample holdcircuit 6. In other words, since holding by the sample hold circuit 6can be performed at the timing indicated as a CURRENT VALUE UPDATE inFIGS. 3A-3D, even in the case of an independent bit input during thetime t3 or t5, the drive current of the light emitting element 1 can becontrolled so as to obtain a normal optical output.

The case wherein the output signal of the current-voltage convertingcircuit 3 has risen to the reference level when a “1” has continued fortwo (2) bits or longer will now be described below. The continuousnumber of “1” bits to be detected is set in accordance with therelationship between the speed of the input data (DATA) and the responsecharacteristics of the light receiving element 2. For example, when “1”has continued for three (3) bits or longer, and a light receivingelement 2 having response characteristics which increase to the setvalue is used, as described above, the “1” continuous signal detectingcircuit 8 may operate so that the analog switching circuit 7 is switchedon when three continuous “1” bits have been detected.

FIG. 4 is a block diagram of an optical transmitter in accordance with asecond embodiment of the present invention. The optical transmittershown in FIG. 4 includes a light emitting element 11, such as asemiconductor laser, a light receiving element 12 such as a photodiode,a current-voltage converting circuit (I/V) 13, an APC amplifier 14, adrive circuit 15, a sample hold circuit 16, an analog switching circuit17, a “1” continuous signal detecting circuit 18, and a peak detectingcircuit 19 including a capacitor C3 and a diode D8.

In accordance with the second embodiment of the present invention, in amanner similar to the embodiment shown in FIG. 1, when, for example, atleast two (2) continuous bits of data (DATA) that are input as “1” aredetected by the “1” continuous signal detecting circuit 18, the analogswitching circuit 17 is switched on, and the output signal of the APCamplifier 14 is input to the sample hold circuit 16 and held. The valueheld by the sample hold circuit 16 is input to the drive circuit 15 as acurrent control signal to control the drive current supplied to thelight emitting element 11 in accordance with the input data (DATA). Theoptical output of the light emitting element 11 corresponding to thedrive current is monitored by the light receiving element 12.

The peak detecting circuit 19 is provided between the current-voltageconverting circuit 13 and the APC amplifier 14. The peak detectingcircuit 19 detects the peak value of the output signal of thecurrent-voltage converting circuit 13 and inputs the detected peak valueto the APC amplifier 14. The output signal of the current-voltageconverting circuit 13 is temporarily held by the peak detecting oncircuit 19. Temporarily holding the output signal of the current-voltageconverting circuit 13 stabilizes the output signal of the APC amplifier14, which is input via the analog switching circuit 17 to the samplehold circuit 16, thereby preventing an operational error when thecurrent control signal that is held in the sample hold circuit 16 isupdated.

FIGS. 5A-5D are diagrams explaining the operation of the opticaltransmitter in accordance with the second embodiment of the presentinvention. More specifically, FIG. 5A illustrates the data (DATA) inputinto the drive circuit 15; FIG. 5B illustrates the output signal of the“1” continuous signal detecting circuit 18; FIG. 5C illustrates theoutput signal of the peak detecting circuit 19; and FIG. 5D illustratesthe drive current applied from the drive circuit 15 to the lightemitting element 11.

As shown in FIGS. 5A-5D, when two (2) bits of input data (DATA) continueas “1,” as during times t0 and t1 in FIG. 5A, drive current is suppliedto the light emitting element 11, as shown in FIG. 5D, and thecorresponding optical output is detected by the light receiving element12. At this time, the output signal of the current-voltage convertingcircuit 13 gradually increases in accordance with the responsecharacteristics of the light receiving element 12. Thus, the outputsignal of the peak detecting circuit 19 also gradually increases, asshown in FIG. 5C, to the set value indicated by the broken line duringtime t2, for example.

Furthermore, as shown in FIG. 5B, when the “1” continuous signaldetecting circuit 18 detects two (2) continuous “1” bits, it outputs adetection signal during time t2. The analog switching circuit 17 is thusswitched on, and the output signal of the peak detecting circuit 19shown in FIG. 5C at this time is held by the sample hold circuit 16.More particularly, during the “on” interval of the analog switchingcircuit 17, which is indicated as a CURRENT VALUE UPDATE INTERVAL inFIGS. 5A-5D, the current control signal is updated with respect to thedrive circuit 15. In this case, the peak value of the output signal ofthe current-voltage converting circuit 13 is held by the peak detectingcircuit 19, so that holding in the sample hold circuit 16 can bereliably performed.

FIG. 6 is a block diagram of an optical transmitter in accordance with athird embodiment of the present invention. The optical transmitter shownin FIG. 6 includes a light emitting element 21 such as a semiconductorlaser, a light receiving element 22 such as a photodiode, acurrent-voltage converting circuit (I/V) 23, an APC amplifier 24, adrive circuit 25, a sample hold circuit 26, an analog switching circuit27, a “1” continuous signal detecting circuit 28, and peak detectingcircuits 29 and 30.

In accordance with the third embodiment of the invention, the peakdetecting circuit 29 operates in the same manner as the peak detectingcircuit 19 in the third embodiment shown in FIG. 4. The peak detectioncircuit 30 detects the peak values of the input data (DATA) and uses thedetected value thereof as the reference value for the APC amplifier 24.By inputting the output signal of the current-voltage converting circuit23 and the reference value to the APC amplifier 24 via the respectivepeak detecting circuits 29 and 30, the error portion of the peakdetecting circuit 29 can be offset, and the optical output of the lightemitting element 21 can be monitored with better accuracy.

FIG. 7 is a block diagram of an optical transmitter in accordance with afourth embodiment of the present invention. The optical transmitter inaccordance with the fourth embodiment of the invention includes a lightemitting element 31 such as a semiconductor laser, a light receivingelement 32 such as a photodiode, a current-voltage converting circuit(I/V) 33, an APC amplifier 34, a drive circuit 35, a sample hold circuit36 which includes an analog switch circuit, a “1” continuous signaldetecting circuit 38, a peak detecting circuit 39, and a delay circuit40.

The fourth embodiment of the present invention is similar to the thirdembodiment of the invention shown in FIG. 4 and further includes thedelay circuit 40 between the “1” continuous signal detecting circuit 38and the analog switch circuit 36. The delay circuit 40 is alsoapplicable to the embodiments of the invention shown in FIG. 1 and FIG.6. The delay circuit 40 may, for example, be a delay circuit as shown inFIG. 8. However, various other types of delay circuits may be used.

As shown in FIG. 8, the delay circuit 40 is preferably a shift registerincluding a plurality of flip-flops 41-1 through 41-n which arevertically connected, with a clock signal CLK input to the respectiveclock terminals C of each flip-flop 41-1 through 41-n, and a detectionsignal from the “1” continuous signal detecting circuit 38 input to thedata terminal D of the initial flip-flop 41-1. The delay time of theshift register delay circuit 40 can be selected in accordance with thespeed of the clock signal CLK and the number of connections of theflip-flops 41-1 through 41-n.

As shown in FIG. 7, the output signal of the current-voltage convertingcircuit 33 is input to the peak detecting circuit 39 and its peak valueis detected. However, as shown in FIG. 5C, there is a deviation in thetime until the set value indicated by the broken line is reached, andthere is also deviation in the timing at which two (2) continuous “1”bits are detected in the case of the embodiment shown in FIG. 2.

Therefore, the “1” continuous detection signal is delayed by the delaycircuit 40 until the output signal of the current-voltage convertingcircuit 33 corresponding to the optical output by the light emittingelement 31 is obtained. When the output signal of the peak detectingcircuit 39 has been stabilized, the output signal of the APC amplifier34 is held in the sample hold circuit 36 and is used as a currentcontrol signal that is input to the drive circuit 35. The operation ofthe other elements shown in FIG. 7 is similar to the operation of likeelements in the embodiments described above, and an explanation of theselike elements will not be repeated here.

FIG. 9 is a block diagram of an optical transmitter in accordance with afifth embodiment of the present invention. As shown in FIG. 9, theoptical transmitter includes a light emitting element 51 such as asemiconductor laser, a light receiving element 52 such as a photodiode,a current-voltage converting circuit (I/V) 53, an APC amplifier 54, adrive circuit 55, a digital hold circuit 56, an up/down counter (U/D)57, a “1” continuous signal detecting circuit 58, a peak detectingcircuit 59, and a digital-to-analog (D/A) converter 60.

In accordance with the fifth embodiment of the present invention, thedigital hold circuit 56, which corresponds to the sample hold circuit inthe above-described embodiments, comprises the up/down counter 57 andthe D/A converter 58. As shown in FIG. 9, the detection signal from the“1” continuous signal detecting circuit 58 is used as an enable signalof the up/down counter 57. An up-count or a down-count is performed incorrespondence to the output signal of the APC amplifier 54 when, forexample, two (2) continuous “1” bits are detected in the input data(DATA).

For example, when the output signal of the peak detecting circuit 59 ishigher than the reference value during detection of two (2) continuous“1” bits, the output signal of the APC amplifier 54 causes the up/downcounter 57 to count down, while the up/down counter 57 counts up inaccordance with the output signal of the APC amplifier 54 when theoutput signal of the peak detecting circuit 59 is lower than thereference value. In other words, the up/down counter 57 counts down whenthe optical output of the light emitting element 51 exceeds a set value,and counts up when the optical output of the light emitting element 51is lower than the set value.

Accordingly, since the current control signal that is obtained byconverting the count content of the up/down counter 56 into an analogsignal by the D/A converter 60 is added to the gate of the transistor Q7of the drive circuit 5 shown in FIG. 2, the optical output of the lightemitting element 51 can be controlled to be a set value.

FIG. 10 is a diagram explaining the “1” continuous signal detectingcircuit in accordance with embodiments of the present invention. Asshown in FIG. 10, the “1” continuous signal detecting circuit is a shiftregister is comprising of a plurality of vertically connected flip-flops61-1 through 61-n. A clock signal CLK is input to respective clockterminals C of the flip-flops 61-1 through 61-n, input data (DATA) isinput into a data terminal D of the initial flip-flop 61-1, and theoutput signal Q of each respective flip-flop 61-1 through 61-n is inputinto an AND circuit 62.

When, for example, the clock signal CLK is synchronized with the bit ofthe input data (DATA), and two (2) continuous “1” bits are detected, thetwo flip-flops 61-1 and 61-2 are used in the “1” continuous bitdetecting circuit, since the output terminals Q of the flip-flops 61-1and 61-2 are both “1” because of the two continuous “1” bits, the outputof the AND circuit 62 becomes “1.” In other words, in the case of two(2) or more continuous “1” bits, continuous “1” detection signals areobtained. When a “0” bit is input, the output signal of the AND circuit62 becomes “0.”

Counting up or counting down by the up/down counter 57 is performedusing the detection signal of the “1” continuous bit detecting circuitas the enable signal of the up/down counter 57. Furthermore, when three(3) or more continuous “1” bits are detected, three or more verticallyconnected flip-flops can be used. Of course, the “1” continuous signaldetecting circuit shown in FIG. 10 can also be used as the “1”continuous signal detecting circuits 8, 18, 28, and 38 in theembodiments described above.

FIG. 11 is a block diagram of an optical transmitter in accordance witha sixth embodiment of the present invention. As shown in FIG. 11, theoptical transmitter includes a light emitting element 71 such as asemiconductor laser, a light receiving element 72 such as a photodiode,a current-voltage converting circuit (I/V) 73, a comparator 74 to detectlight deterioration, a drive circuit 75, a flip-flop 76, and a “1”continuous signal detecting circuit 78.

The optical output of the light emitting element 71 is monitored by thelight receiving element 72, and, when the optical output does not reachthe desired optical output even when the drive current from the drivecircuit 75 is controlled, it is judged that deterioration of the lightemitting element 71 has occurred. In this case, when an inexpensivephotodiode having response characteristics that are unable to follow theoptical output of the light emitting element 71 is used as the lightreceiving element 72, even if the optical output corresponding to oneisolated “1” bit in the input data (DATA) is monitored, the outputsignal of the current-voltage converting circuit 73 does not rise to thespecified value, as described with reference to FIG. 3 and FIG. 5.Therefore, there is a possibility that the light emitting element 71will be judged to have deteriorated.

However, in accordance with embodiments of the present invention, when acontinuous “1” in the input data (DATA) is detected by the “1”continuous signal detecting circuit 78, it is determined whether or notthe light emitting element 71 has deteriorated. More particularly, theoutput signal of the current-voltage converting circuit 73, whichconverts the output current of the light receiving element 72 tovoltage, and the reference value Vr input to the light deteriorationdetecting comparator 74, are compared. A comparison output signal of “0”is input into the data terminal D of the flip-flop 76 when the outputvalue of the current-voltage converting circuit 73 exceeds the referencevalue Vr, and a “1” is input to the data terminal D of the flip-flop 76when the output value of the current-voltage converting circuit 73 islower than the reference value Vr. The detection signal of the “1”continuous signal detecting circuit 78 is input to the clock terminal Cof the flip-flop 76.

Thus, when the light emitting element 71 is in a normal status, theoutput signal of the current-voltage converting circuit 73 does not riseto the specified level in the case where the input data (DATA) is anindependent “1” bit. Further, even if a “1” signal indicatingdeterioration is output from the light deterioration detectingcomparator 74, because the flip-flop 76 is not set, an optical outputdeterioration signal is not produced. However, in the case of continuous“1” bits, that is, when the “1” continuous signal detecting circuit 78detects two (2) continuous “1” bits in the input data (DATA), to judgethat deterioration has occurred with two (2) continuous bits, if theoutput signal of the current-voltage converting circuit 73 does notreach the set value when there are two (2) continuous “1” bits aredetected, the flip-flop 76 is set, an optical output deteriorationsignal is transmitted, and a deterioration alarm of the light emittingelement 71 is activated.

In the embodiments described above, the drive circuit 75 may be any ofvarious types of drive circuits. Further, when the responsecharacteristics of the light receiving element 72 are relatively slow,the “1” continuous signal detecting circuit 78 can be designed tooperate to determine whether or not there is deterioration of the lightemitting element 71 when three continuous “1” bits are detected.

FIG. 12 is a block diagram of an optical transmitter in accordance witha seventh embodiment of the present invention. As shown in FIG. 12, theoptical transmitter includes a light emitting element 81 such as asemiconductor laser, a light receiving element 82 such as a photodiode,a current-voltage converting circuit (I/V) 83, a comparator to detectoptical deterioration 84, a drive circuit 85, a sample hold circuit 86,a “1” continuous signal detecting circuit 88, peak detecting circuits 89and 90, a comparator 91 to detect optical deterioration, and a flip-flop92.

In accordance with the seventh embodiment of the present invention, thepeak detecting circuit 90, the comparator 91 to detect opticaldeterioration, and the flip-flop 92 are provided addition to theelements shown in the embodiment of FIG. 4, allowing the deteriorationof the light emitting element 81 to be detected thereby. Furthermore,the monitoring of the optical output of the light emitting element 81 bythe light receiving element 82, and the controlling of the drive circuit85 in accordance with a current control signal so that the opticaloutput remains constant, are performed in a manner similar to theembodiments shown in FIG. 1, FIG. 4, FIG. 6, and FIG. 7, and anexplanation of these elements and their associated functions will not berepeated here.

In order to detect deterioration of the optical output generated by thelight emitting element 81, the peak value of the output signal of thecurrent-voltage converting circuit 83 is detected by the peak detectingcircuit 90 and is compared with a reference value Vr by the opticaldeterioration detecting comparator 91 in a stabilized state. When thepeak value is less than the reference value Vr, a “1” indicatingdeterioration of the light emitting element 81 is output, and otherwisea “0” is output. When a continuous “1” is not detected in the input data(DATA) by the “1” continuous signal detecting circuit 88, deteriorationevaluation is not performed. However, when a continuous “1” is detectedby the “1” continuous signal detecting circuit 88, the detected signalis input to the clock terminal C of the flip-flop 92. Accordingly, whenthe output signal of the for light deterioration detecting comparator 91that is input into the data terminal D of the flip-flop 92 is “1” theflip-flop 92 is set, and the optical output deterioration signal of “1”is output from the output terminal Q.

The present invention is not limited to the embodiments described above,and various additions and alterations, as well as various combinationsof the embodiments are possible. For example, embodiments of the presentinvention may operate such that the output signal of the current-voltageconverting circuit is converted into a digital signal which undergoesdigital processing.

As described above, in accordance with embodiments of the presentinvention, by providing a “1” continuous signal detecting circuit todetect continuous “1” bits in the input data (DATA), a conventionalphotodiode having a comparatively large capacity can be used as thelight receiving element to monitor the light emitting element, such as asemiconductor laser or light emitting diode. More specifically, by usingthe detection of continuous “1” bits from the “1” continuous signaldetection circuit, an update timing of the current control signal or adeterioration evaluation timing of the light emitting element can beperformed. Accordingly, the advantage of cost reduction in the lighttransferring circuit can be achieved.

Although a few preferred embodiments of the present invention have beenshown and described, it will be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

What is claimed is:
 1. An optical transmitter circuit, comprising: alight emitting element to convert input data into an optical output; alight receiving element to receive the optical output of the lightemitting element and to output a current corresponding to the opticaloutput; a current-voltage converting circuit to convert the outputcurrent of the light receiving element into a voltage and to output avoltage signal; an amplifier to compare the output voltage signal of thecurrent-voltage converting circuit and a reference signal and to outputa differential output signal; a hold circuit to hold the output signalof the amplifier and to form a current control signal; a drive circuitto receive the current control signal from the hold circuit and tosupply drive current to the light emitting element in accordance withthe current control signal; and a “1” continuous signal detectingcircuit to detect a specified number of continuous “1” bits in the inputdata and to perform an updating operation on the output signal of theamplifier held in the hold circuit.
 2. An optical transmitter circuit asrecited in claim 1, wherein the hold circuit comprises: an analogswitching circuit to receive the output signal of the amplifier and adetection signal of the “1” continuous signal detecting circuit, and toswitch on in response to the “1” continuous signal detecting circuitdetecting the specified number of continuous “1” bits; and a peakdetecting circuit to detect and hold a peak value of the output signalof the amplifier that is input via the analog switching circuit.
 3. Anoptical transmitter circuit as recited in claim 1, further comprising apeak detecting circuit to detect a peak value of the output signal ofthe current-voltage converting circuit and to input the peak value tothe amplifier.
 4. An optical transmitter circuit as recited in claim 1,further comprising: a first peak detecting circuit to detect a peakvalue of the output signal of the current-voltage converting circuit andto output the peak value to the amplifier; and a second peak detectingcircuit to detect a peak value of the input data and to output the peakvalue of the input data to the amplifier as a reference value.
 5. Anoptical transmitter circuit as recited in claim 1, wherein the holdcircuit comprises: an up/down counter to count up or down correspondingto an output signal of the amplifier, in response to the “1” continuoussignal detecting circuit detecting the specified number of continuous“1” bits; and a D/A converter to convert the contents of the count ofthe up/down counter to an analog signal and to input the contents of thecount of the up/down counter to the drive circuit as an analog currentcontrol signal.
 6. An optical transmitter circuit as recited in claim 5,wherein the up/down counter counts down in response to the output of thelight emitting element exceeding a set value and counts up in responseto the output of the light emitting element being lower than the setvalue.
 7. An optical transmitter circuit, comprising: a light emittingelement to convert input data into an optical output; a light receivingelement to receive the optical output of the light emitting element andto output a current corresponding to the optical output; acurrent-voltage converting circuit to convert the output current of thelight receiving element into a voltage; a light deterioration detectingcomparator to detect deterioration in the light receiving element bycomparing the output signal of the current-voltage converting circuitwith a reference value and to output a deterioration detection signal; a“1” continuous signal detecting circuit to detect a specified number ofcontinuous “1” bits in the input data and to output a detection signal;and a flip-flop to hold the deterioration detection signal from thelight deterioration detection comparator in accordance with thedetection signal from the “1” continuous signal detecting circuit.
 8. Anoptical transmitter circuit as recited in claim 7, further comprising:an amplifier to receive the output voltage of the current-voltageconverting circuit; a drive circuit to drive the light emitting element;and a hold circuit to hold the output signal of the amplifier inaccordance with the detection signal from the “1” continuous signaldetecting circuit and to input a current control signal to the drivecircuit of the light emitting element.
 9. An optical transmitter circuitas recited in claim 8, further comprising a delay circuit to delay thedetection signal from the “1” continuous signal detecting circuit and toinput the delayed detection signal to the hold circuit and flip-flop.10. An optical transmitter including a light emitting element to convertinput data to an optical output, comprising: a current generator togenerate a drive current to drive the light emitting element; acontinuous signal detecting circuit to detect a predetermined number ofcontinuous “1” bits in the input data and to output a control signal tothe current generator to control the drive current in response todetecting the predetermined number of “1” bits, wherein the currentgenerator comprises: a light receiving element to receive the opticaloutput of the light emitting element and to output a currentcorresponding to the optical output; a current-voltage convertingcircuit to convert the output current of the light receiving elementinto a voltage and to output a voltage signal; a drive circuit to supplydrive current to the light emitting element; and a current controlsignal generator to receive the voltage signal output by thecurrent-voltage converting circuit and the control signal output by thecontinuous signal detecting circuit and to generate a current controlsignal to control the drive current supplied to the light emittingelement by the drive circuit.
 11. An optical transmitter as recited inclaim 10, wherein the current control signal generator includes aswitching circuit, and the control signal output by the continuoussignal detecting circuit controls switching of the switching circuit toupdate the current control signal.
 12. An optical transmitter as recitedin claim 10, wherein the current control signal generator includes anup/down counter to count up or down in response to the control signalgenerated by the continuous signal detecting circuit, wherein a count ofthe up/down counter controls the drive current.
 13. An opticaltransmitter as recited in claim 11, wherein the current control signalgenerator further comprises a delay circuit between the continuoussignal detecting circuit and the switching circuit to delay the controlsignal from the continuous signal detecting circuit.
 14. An opticaltransmitter as recited in claim 11, wherein the current control signalgenerator comprises: a comparator to compare the output voltage signalof the current-voltage converting circuit and a reference signal and tooutput a difference signal; and a sample and hold circuit to receive thedifference signal output by the comparator in response to the switchingcircuit being switched on, to hold the difference signal output by thecomparator, and to input the held difference signal to the drive circuitas the current control signal.
 15. An optical transmitter circuit,comprising: a light emitting element to convert input data into anoptical output; a light receiving element to receive the optical outputof the light emitting element and to output a current corresponding tothe optical output; a current-voltage converting circuit to convert theoutput current of the light receiving element into a voltage and tooutput a voltage signal; an amplifier to compare the output voltagesignal of the current-voltage converting circuit and a reference signaland to output a differential output signal; a hold circuit to hold theoutput signal of the amplifier and to form a current control signal; adrive circuit to receive the current control signal from the holdcircuit and to supply drive current to the light emitting element inaccordance with the current control signal; and means for detecting aspecified number of continuous “1” bits in the input data and forperforming an updating operation on the output signal of the amplifierheld in the hold circuit.
 16. An optical transmitter circuit,comprising: a light emitting element to convert input data into anoptical output; a light receiving element to receive the optical outputof the light emitting element and to output a current corresponding tothe optical output; a current-voltage converting circuit to convert theoutput current of the light receiving element into a voltage; means fordetecting deterioration in the light receiving element by comparing theoutput signal of the current-voltage converting circuit with a referencevalue and for outputting a deterioration signal; means for detecting aspecified number of continuous “1” bits in the input data and foroutputting a detection signal; and means for holding the deteriorationsignal in accordance with the detection signal.
 17. An opticaltransmitter including a light emitting element to convert input data toan optical output, comprising: a current generator to generate a drivecurrent to drive the light emitting element; and a continuous signaldetecting circuit to detect a predetermined number of continuous “1”bits in the input data and to output a control signal to the currentgenerator to control the drive current in response to detecting thepredetermined number of “1” bits, wherein the current generatorcomprises: a light receiving element to receive the optical output ofthe light emitting element and to output a current corresponding to theoptical output; a current-voltage converting circuit to convert theoutput current of the light receiving element into a voltage and tooutput a voltage signal; a drive circuit to supply drive current to thelight emitting element; and means for generating a current controlsignal to control the drive current supplied to the light emittingelement by the drive circuit, in accordance with the voltage signaloutput by the current-voltage converting circuit and the control signaloutput by the continuous signal detecting circuit.